a) Field of the Invention
The invention relates to a method for obtaining an output ion current substantially comprised of a single ionic species, in which ions formed during the ionization of a source gas in an ionization region and/or ions extracted from the ionization region are allowed to react in a region, in which is disposed a source gas, until substantially only one or several source ionic species, which do not react with the source gas, are present.
b) Description of Related Prior Art
Such a method is disclosed for example in AT 001 637 U1. In this document a method for obtaining an ion current is described, which is substantially comprised of H3O+ ions. For this purpose water vapor is ionized by means of an ion source in an ionization region, whereby various ions are formed (O+, OH+, H+, H2+, . . . ). These ions are extracted by means of a weak electric field into a region located outside of the ionization region and are kept in this region, in which H2O is present at a pressure above 0.01 Torr, until those ions, initially differing from H3O+, have also been converted into H3O+ ions in secondary reactions. In this region and/or in a region adjoining thereon, the ion current is furthermore guided through an electric field, whose field strength is of adequate magnitude such that H3O+.(H2O)n cluster ions, formed through association reactions between two successive collisions with neutral collision partners have gained sufficient kinetic energy in order for these collisions to be largely dissociative. The build-up of such cluster ions is thereby prevented or largely canceled. To improve these dissociation reactions, an additional gas, such as Ar, Kr or N2, which serves as a collision partner for the cluster ions but does not enter into chemical reactions with the H3O+ ions can also be added to the H2O.
Such an ion current, substantially comprised of H3O+, can be utilized in particular as a primary ion current for the chemical ionization of a sample gas through proton transfer reactions, in order to analyze the ions formed of the sample gas mass-spectrometrically. This proton transfer reaction mass spectrometry, referred to as PTR-MS, is described in AT 001 637 U1 and the references cited therein. Involved here is a special type of ion molecule reaction mass spectrometry (IMR-MS), which is also described in AT 001 637 U1 and the references cited therein.
AT 406 206 B, moreover, discloses a method with a process sequence analogous to that known from AT 001 637 for obtaining an ion current, substantially comprised of NH4+ ion. As the source gas for this purpose ammonia (NH3) is ionized and, after extraction from the ionization region, the ions formed are allowed to rest in a region at an ammonia pressure above 0.01 Torr (1.33 Pascal) until an ion current, substantially only comprised of NH4+ ions, is formed (and for the prevention or cancellation of the build-up of cluster ions, again, an electric field strength sufficient for inducing collisions is applied).
From AT 403 214 B is furthermore known to introduce different source gases into an ion source and through a filter device to filter all primary ionic species, generated in the ion source from various neutral atoms or molecules of the source gases, except one primary ionic species. The remaining primary ionic species is introduced into the reaction chamber. In the reaction chamber it is allowed to react with a sample gas, and the reaction products formed through ion-molecule reactions (for example proton transfer reactions) are analyzed in a mass spectrometer. Of disadvantage here is the additionally required mass spectrometer forming the filter device.
Obtaining output ion currents through the methods disclosed in AT 001 637 U1 or AT 406 206 B comprised of only a single ionic species without such mass-spectrometric filtering, such as is known from AT 403 214 B, is only possible for some ionic species, in particular for H3O+ ions, NH4+ ions and H3+ ions. Only in a few source gases output ion currents are formed in the manner described in these two documents, which substantially are comprised of only a single ionic species. Such source gases are described in the literature as “CI Reagent Gases”.
EP 000 865 A1 describes the analysis of a sample gas which, for this purpose, is ionized through ion-molecule reactions. Chemical ionization of the sample gas takes place in a chamber (conventionally also referred to as “drift tube”), here described as ionization chamber. A partially ionized primary gas from an ion source is introduced into the ionization chamber, which is here implemented as a gas discharge chamber. In addition to the sample gas, into the ionization chamber is also introduced a reactant gas which reacts with the ions entering the ionization chamber from the ion source and, on his part, ionizes the sample gas. Consequently, in the ionization chamber is present a mixture of the more or less ionized components of the primary gas, reactant gas and sample gas. Obtaining an output ion current comprised substantially of only a single ionic species, is not disclosed in this document. To the outlet opening of the ionization chamber are conducted the ionized primary particles as well as also the reactant gas and sample ions.